Method of producing thin films of compound I-III-VI,promoting the incorporation of III elements in the film

ABSTRACT

The invention relates to a method of producing thin films of compound CIGS by means of electrodeposition. According to the invention, a surface-active compound, such as dodecyl sodium sulphate, is added to an electrolysis bath solution in order to promote the incorporation of gallium in the CIGS films.

The present invention relates to the production of semiconductors of theI-III-VI₂ type in thin film form, especially for the design of solarcells.

I-III-VI₂ compounds of the CuIn_((1-x))Ga_(x)Se_(y)S_((2-y)) type (wherex is substantially between 0 and 1 and y is substantially between 0 and2) are regarded as very promising and could constitute the nextgeneration of thin-film photovoltaic cells. These compounds have a widedirect bandgap of between 1.05 and 1.6 eV, which allows solar radiationin the visible to be strongly absorbed.

Record photovoltaic conversion efficiencies have been achieved bypreparing thin films by evaporation on small areas. However, evaporationis difficult to adapt to the industrial scale because of problems ofnonuniformity and low utilization of raw materials. Sputtering is bettersuited to large areas, but it requires very expensive vacuum equipmentand precursor targets.

There is therefore a real need for alternative, low-costatmospheric-pressure, techniques. The technique of thin-film depositionby electrochemistry, in particular by electrolysis, appears to be a veryattractive alternative. The advantages of this deposition technique arenumerous, and in particular the following:

deposition at ambient temperature and ambient pressure in anelectrolysis bath;

possibility of handling large areas with high uniformity;

ease of implementation;

low installation and raw material costs (no special forming operation,high level of material utilization); and

great variety of possible deposit shapes due to the localized nature ofthe deposit on the substrate.

Despite extensive research in this field, the difficulties encounteredrelate to how to control the quality of the electrodeposited precursors(composition and morphology) and, more particularly, the difficulty ofinserting metals such as gallium or aluminum (elements III) whoseelectrodeposition potential is very cathodic.

I-III-VI₂ compounds in which:

the element I corresponds to Cu;

the element III corresponds to In and to Ga and/or Al; and

the element VI corresponds to Se and/or S, will be denoted hereafter bythe abbreviation CIGS.

Moreover, the term “film” is understood to mean a thin layer depositedon a substrate, and the term “precursor film” is understood to mean athin layer of overall composition close to I-III-VI₂ and obtaineddirectly after deposition by electrolysis, with no optional subsequenttreatment.

As regards pure electrodeposition of CIGS (with no evaporation step),the morphology and the composition of the precursors are very difficultto control, as the following documents indicate:

“One step electrodeposited CuIn_(1-x)Ga_(x)Se₂ thin films: structure andmorphology”, M. Fahourme, F. Chraibi, M. Aggour, J. L. Delplancke, A.Ennaoui and M. Lux-Steiner, 17th European Photovoltaic Solar EnergyConference, Oct. 22-26, 2001, Munich, Germany; and

“CuIn_(1-x)Ga_(x)Se₂-based photovoltaic cells from electrodepositedprecursor films”, Materials Research Society Symposium—Proceedings,Volume 668, 2001, pages H8101-H8106, by R. N. Bhattacharya and Arturo M.Fernandes.

The most recent developments have involved an evaporation step after theelectrodeposition, so as to increase the In and Ga contents of theelectrodeposited films. In these developments, especially thosedescribed in document WO 01/78154, the electrodeposition is an actualcodeposition of the elements Cu, In, Ga and Se (in order to obtain aquaternary alloy) and employs a method of deposition in a pH bufferedelectrolytic bath. The buffer solution is composed of sulfamic acid andpotassium biphthalate, forming a buffer of the pHydrion (registeredtrademark) type. Electrodeposited films that have given photovoltaiccells using the hybrid method involving an electrodeposition stepfollowed by an evaporation step have a dendritic morphology of lowdensity.

The present invention aims to improve the situation.

For this purpose, it proposes a method of producing a I-III-VI_(y)compound in thin film form, in which y is close to 2, byelectrochemistry, comprising the following steps:

an electrolysis bath comprising at least one element III dissolved inthe bath and at least two electrodes immersed in the bath is provided,and

a potential difference is applied between the two electrodes in order toinitiate the formation of a thin film of I-III-VI_(y) on the surface ofone of the electrodes.

According to the invention, the electrolysis bath furthermore includesat least one surfactant compound in order to promote the incorporationof the element III into said film.

Advantageously, the element III comprises gallium and/or aluminum.

Preferably, the surfactant compound has a chemical formulaCH₃(CH₂)_(n)O—SO₃—X, where n is greater than or equal to 5 and X is anatomic species such as H, Na, Li or K.

In a preferred embodiment, the surfactant compound comprises sodiumdodecylsulfate.

Alternatively or additionally, the surfactant compound comprises2-butyne-1,4-diol and/or maleic acid and/or succinic acid and/or fumaricacid and/or crotonic acid.

Preferably, the concentration of the surfactant in the electrolysis bathis substantially of the same order of magnitude as the concentration ofgallium and/or aluminum.

Other advantages and features of the invention will become apparent onreading the detailed description below of embodiments given by way ofnonlimiting examples, and from examining the drawings which accompanyit, in which:

FIG. 1 shows schematically a thin film obtained by implementing themethod according to the invention;

FIG. 2 shows schematically an electrolysis bath for implementing themethod according to the invention;

FIG. 3 shows schematically the appearance of a thin film of the priorart, seen from above; and

FIG. 4 shows schematically a sectional view of a thin film of the priorart, being formed.

Referring to FIG. 1, copper diselenide and indium-gallium (as elementIII) layers CO are obtained at ambient pressure and ambient temperatureby the electrodeposition of a thin precursor film of suitablecomposition and morphology on a glass substrate S coated with molybdenumMo.

The electrodeposition is carried out using an acid bath B (FIG. 2),stirred by blades M, containing an indium salt, a gallium salt, a coppersalt and selenium oxide in solution. The concentrations of theseprecursor elements are between 10⁻⁴ and 10⁻² M, where the symbol “M”corresponds to the unit “mole per liter”. The pH of the solution isfixed between 1 and 4.

Three electrodes An, Calif. and REF, including:

a molybdenum electrode Ca on which the thin film is formed byelectrodeposition; and

a mercurous sulfate reference electrode REF, are immersed in the bath B.

The electrical potential difference applied to the molybdenum electrodeis between −0.8 and −1.4 V relative to the reference electrode REF.

Films with a thickness of between 1 and 4 microns are obtained withcurrent densities of between 0.5 and 10 mA/cm².

Under defined composition, solution stirring and potential differenceconditions, it is possible to obtain dense adherent films of homogeneousmorphology, the composition of which is close to the stoichiometriccomposition: Cu (25%); In+Ga (25+ε %) and Se (50%), with an (In+Ga)/Cuatomic ratio slightly greater than 1. It is thus possible to formdeposits on areas of 10×10 cm².

However, the incorporation of gallium in order to form thin CGIS filmsoften poses a problem, both from the standpoint of their morphology andtheir composition. Referring to FIG. 3, the precursor layers CO, beingformed by electrolysis under conventional conditions, exhibitprotuberances PR on the surface, these making a nonzero angle α relativeto the principal plane of the surface of the layer (FIG. 4). Such amorphology of the thin film, which is particularly rough on its surface,is not compatible with the manufacture of photovoltaic cells, whichrequire interfaces that are as parallel and as plane as possible inorder to limit light loss and above all to avoid local short circuits(or shunts).

Furthermore, the volume composition of these deposits is lean in gallium(generally less than 5%) and in any case less than that initiallydesired.

The approach proposed in document WO 01/78154 consists in controllingthe acidity of the electrolysis bath in order to ensure that its pH isstable and, consequently, to promote incorporation of gallium (anelement whose deposition potential is very negative) into the CIGSlayers being formed. For this purpose, the above document provides abuffer solution comprising sulfamic acid and potassium biphthalate inconcentrations that are sufficient to ensure stability of the pH.CuIn_((1-x))Ga_(x)Se₂ films are then obtained with x close to 9%.

In another approach, the present invention proposes to add one or moresurfactant additives to the electrolysis bath in order to form the CIGSfilms. CuIn_((1-x))Ga_(x)Se₂ films obtained by implementing the methodaccording to the invention have a satisfactory morphology and apercentage x of gallium close to, or even greater than theaforementioned 9% value, as will be seen later with reference to apreferred embodiment.

One possible explanation of this improvement in the quality of the filmsby adding surfactants to the bath is the following. The addition of asurfactant compound, acting in the bath by being adsorbed on theelectrode Ca on which the thin film forms, makes it possible to modifythe surface tension at the interface between the thin film being formedand the solution of the bath. Thus, the activation energy for thereaction of incorporating the gallium combined with selenium into thethin film is lowered. The mixing of gallium with other elements, Cu, Inand Se, therefore makes it possible to obtain a homogeneous morphologyof the film, and a composition rich in gallium.

Another possible explanation, in addition to the above one, is that thesurfactants used may furthermore play an inhibiting role in the hydrogenevolution reaction usually observed in electrolysis. This would allowmore cathodic potentials to be applied, thus promoting the incorporationof gallium.

A leveling effect of the surfactants added may also be noted, allowingthe surface of the film being formed to be made plane.

Thus, according to the invention, one or more surfactant additives, forimproving the morphology and/or changing the relative ratios of thevarious electrodeposited elements (Cu—In—Ga—Se), are added to thesolution. It will be understood that their main role is to help in theinsertion of gallium into the precursor layers. The amount of galliumthat can be inserted into the films may vary from 0 to 30% (in atomicpercentage). The concentration of the additives may vary from 10⁻⁵ to10⁻² M.

Given below are various embodiments of the invention, with the followingas surfactant additives:

sodium dodecylsulfate;

2-butyne-1,4-diol;

succinic acid;

fumaric acid; and

maleic acid.

PREFERRED EMBODIMENT Sodium Dodecylsulfate

A typical deposit was produced from an acid bath whose concentrations ofthe precursor elements and of the surfactant CH₃(CH₂)₁₁OSO₃Na were thefollowing:

[CuSO₄]=4.5×10⁻³ M;

[In₂(SO₄)₃]=2.5×10⁻³ M;

[Ga₂(SO₄)₃]=2.5×10⁻³ M;

[H₂SeO₃]=7.5×10⁻³ M;

[CH₃(CH₂)₁₁OSO₃Na]=20×10⁻³ M

The precursors were deposited by a cathodic reaction for a fixedpotential, namely −1.1 V relative to the electrode REF. The currentdensity was −5 mA/cm². TABLE I Analysis of the composition of anelectrodeposited CIGS film from a solution containing sodiumdodecylsulfate. Element at % Cu 20.70 Ga 10.27 Se 50.94 In 18.10

Advantageously, the morphology of the film was very homogeneous.

More generally, it may be indicated that the addition of surfactants offormula CH₃(CH₂)_(n)O—SO₃—X (where n is greater than or equal to 5 and Xis an atomic species such as H, Na, Li or K) gave satisfactory results.

SECOND EMBODIMENT 2-butyne-1,4-diol

A typical deposit was produced from an acid bath whose concentrations ofprecursor elements and of the surfactant HO—CH₂—C≡C—CH₂—OH were thefollowing:

[CuSO₄]=4.5×10⁻³ M;

[In₂(SO₄)₃]=2.5×10⁻³ M;

[Ga₂(SO₄)₃]=2.5×10 M;

[H₂SeO₃]=7.5×10⁻³ M;

[HO—CH₂—C≡C—CH₂—OH]=20×10⁻³ M.

The precursors were deposited by a cathodic reaction with a potentialset at −1.1 V relative to the electrode REF. The current density was −5mA/cm². TABLE II analysis of the composition of a CIGS filmelectrodeposited from a solution containing 2-butyne- 1,4-diol. Elementat % Cu 23.10 Ga 1.80 Se 53.50 In 21.54

The morphology of the film was not very homogeneous. However, nodebonding of the film was observed.

THIRD EMBODIMENT OF THE INVENTION Maleic Acid

A typical deposit was produced from an acid bath whose concentrations ofprecursor elements and of the surfactant HO₂C—CH═CH—CO₂H were thefollowing:

[CuSO₄]=4.5×10⁻³ M,

[In₂(SO₄)₃]=2.5×10⁻³ M,

[Ga₂ (SO₄)₃]=2.5×10⁻³ M,

[H₂SeO₃]=7.5×10⁻³ M,

[HO₂C—CH═CH—CO₂H]=20×10⁻³ M.

The precursors were deposited by a cathodic reaction for a potential setat −1.1 V relative to the electrode REF. The current density was −5mA/cm². TABLE III analysis of the composition of a CIGS filmelectrodeposited from a solution containing maleic acid. Element at % Cu23.32 Ga 3.10 Se 53.32 In 20.26

The morphology of the film was substantially homogeneous.

FOURTH EMBODIMENT Succinic Acid

A typical deposit was produced from an acid bath whose concentrations ofprecursor elements and of the surfactant HO₂—CH₂—CH₂—CO₂H were thefollowing:

[CuSO₄]=4.5×10⁻³ M,

[In₂(SO₄)₃]=2.5×10⁻³ M,

[Ga₂ (SO₄)₃]=2.5×10⁻³ M,

[H₂SeO₃]=7.5×10⁻³ M,

[HO₂—CH₂—CH₂—CO₂H]=20×10⁻³ M.

The precursors were deposited by a cathodic reaction for a potential setat −1.1 V relative to the electrode REF. The current density was −5mA/cm². TABLE IV analysis of the composition of a CIGS filmelectrodeposited from a solution containing succinic acid. Element at %Cu 23.69 Ga 3.99 Se 53.33 In 19.99

The morphology of the film was advantageously homogeneous.

FIFTH EMBODIMENT Fumaric Acid

A typical deposit was produced from an acid bath whose concentrations ofprecursor elements and of the surfactant HO₂—CH—CH—CO₂H were thefollowing:

[CuSO₄]=4.5×10⁻³ M,

[In₂(SO₄)₃]=2.5×10⁻³ M,

[Ga₂ (SO₄)₃]=2.5×10⁻³ M,

[H₂SeO₃]=7.5×10⁻³ M,

[HO₂—CH—CH—CO₂H]=20×10⁻³ M.

The precursors were deposited by a cathodic reaction for a potential setat −1.1 V relative to the electrode REF. The current density was −5mA/cm². TABLE V analysis of the composition of a CIGS filmelectrodeposited from a solution containing fumaric acid. Element at %Cu 24.54 Ga 2.85 Se 52.60 In 20.00

The morphology of the film was substantially homogeneous.

More generally, the additive within the meaning of the invention may bea surfactant compound taken from the following two classes:

the surfactant compounds, the molecule of which contains the X—SO₃—Y orZ-SO₂-Z′ group, in which:

-   -   Y is an element taken from H, Na, Li, K;    -   X is an unsaturated (ethylenic, aromatic or acetylenic) group        that may contain hetero atoms, with any number of carbon atoms,        or else a saturated group that may contain hetero atoms;    -   Z and Z′ are saturated or unsaturated groups that may contain        hetero atoms (S, N or the like); and

compounds whose molecule possesses at least one polar group: —OH—COOH,—S (or other hetero atom) and/or an unsaturated group: alkene, alkyne,aromatic (with or without a hetero atom), allowing the molecule to beadsorbed during electrodeposition.

Each compound of one of the two families may be used by itself or as amixture. The same compound may belong to both families (if it possessesat least one unsaturated group and at least one SO₂ group).

It should be pointed out that these surfactant compounds differ from theusual organic solvents whose solvation role acts only on the solution ofthe bath. They also differ from the organic additives introduced intothe electrolysis bath for stabilizing the pH.

The surfactant compounds described above may be easily used for any typeof electrolysis bath for the electrodeposition of I-III-VI systems suchas Cu—In—Ga—Al—Se—S.

The surfactants allowing gallium to be inserted into the precursor filmsthus make it possible to solve several difficulties described in theprior art (poor control of the morphology, of the composition of theprecursors, in particular as regards the gallium content, and thedifficulty of extending to large areas).

Of course, the present invention is not limited to the embodimentdescribed above by way of example, rather it extends to other variants.

Thus, it will understood that aluminum, as element III, posessubstantially the same problems of incorporation into the Cu—In—Al—Sefilms as gallium. In this regard, the invention applies also to theproduction of such films. Moreover, indium is usually introduced inexcess into the solution of the bath in order to promote itsincorporation into the film, indium combining, as element III, withselenium. It may be pointed out that the addition of surfactants to thebath ought also to promote the incorporation of indium as element III,into the film.

Moreover, it should also be pointed out that crotonic acid, assurfactant additive, has also provided satisfactory results.

1-10. (canceled)
 11. A method of producing a I-III-VI_(y) compound inthin film form, in which y is close to 2, by electrochemistry,comprising: a) providing an electrolysis bath comprising at least oneelement III compound dissolved in the electrolysis bath and at least twoelectrodes immersed in the electrolysis bath; and, b) applying apotential difference between the two electrodes to initiate formation ofa thin film of I-III-VI_(y) on the surface of one of the electrodes,wherein the electrolysis bath further comprises at least one surfactantto promote incorporation of the element III compound into the film. 12.The method of claim 11, wherein the element III compound comprisesgallium or aluminum.
 13. The method of claim 11, wherein the surfactanthas a chemical formula CH₃(CH₂)_(n)O—SO₃—X, where n is greater than orequal to 5 and X is an atomic species selected from the group consistingof H, Na, Li and K.
 14. The method of claim 13, wherein the surfactantcomprises sodium dodecylsulfate.
 15. The method of claim 11, wherein thesurfactant comprises 2-butyne-1,4-diol.
 16. The method of claim 11,wherein the surfactant comprises maleic acid.
 17. The method of claim11, wherein the surfactant comprises succinic acid.
 18. The method ofclaim 11, wherein the surfactant comprises fumaric acid.
 19. The methodof claim 11, wherein the surfactant comprises crotonic acid.
 20. Themethod of claim 12, wherein the surfactant in the electrolysis bath isin a concentration substantially of the same order of magnitude as aconcentration of gallium or a concentration of aluminum in theelectrolysis bath.